It is well known that elastomeric polyurethanes and polyurethane foams are produced by means of a polyaddition process, by reacting a polyol with an organic polyisocyanate, usually in the presence of blowing agents, activating agents such as tertiary amines and/or organometallic compounds, emulsifiers, and so forth.
It is also well known that in order to improve the physical characteristics of polyurethanes, such as their ageing resistance, their elastic modulus, and the dependence of this latter on temperature, a technique known from the prior art consists in dispersing polymer particles through the polymeric matrix. In general, the polymer particles are dispersed into the polyol before it is reacted with the polyisocyanate.
British patent No. 1,040,452 teaches the preparation of the polymer/polyol dispersion by polymerizing the ethylenically unsaturated monomer, from which originates the polymer, directly within the polyol. For that purpose, the ethylenically unsaturated monomer is dissolved or dispersed into the polyol in an amount within the range of from 1 to 60% by weight, and then subsequently polymerized while it is mixed with the polyol.
The polymerization is carried out in the presence of the usual initiators, which can be peroxides, such as lauryl-peroxide, benzoyl-peroxide, di-cumyl-peroxide, and so forth; the nitrogen-containing initiators, such as azo-bis-isobutyronitrile; the redox systems; high-energy radiation, and so forth.
According to said British patent No. 1,040,452, any ethylenically unsaturated monomer containing one or more double bond(s) can be used for producing the polymer particles.
Examples of such monomers are: styrene and vinyl aromatic monomers in general, olefinic or di-olefinic hydrocarbons, such as ethylene, propylene, 1-butylene, 2-hexene, 1,3-butadiene, 2-pentene, etc.; vinyl esters, such as vinyl acetate or in propionate,; vinyl halides, such as vinyl chloride, vinylidene chloride, etc.; acrylic acid, methacrylic acid and their esters or nitriles such as methacrylates, acrylates, acrylonitrile and so forth.
These monomers may be used alone or mixed with each other.
When polyurethanes filled with polymers are prepared starting from a polyol wherein the polymer is dispersed, one of the necessary conditions is that the polymer particles which are formed be homogeneously and stably dispersed throughout the polyol, as to form an "organosol".
Another necessary condition for preparing polymer-filled polyurethanes, useful for the required applications, is that the polyol containing the dispersed polymer have a not too high viscosity, e.g., not higher than 10,000 mnPa.times.sec., so that it can be easily treated with the organic polyisocyanate, both as such or as an isocyanate-polyol prepolymer, in the usual equipment and by means of the usual techniques as usually employed for obtaining polymeric particles homogeneously dispersed throughout the mass.
The processes known from the prior art for obtaining the homogeneous dispersions of the polymer into the polyol by means of an "in situ" polymerization, allowed one to obtain satisfactory results as to the processability and the physical properties of the obtained product, in the case of some ethylenically unsaturated monomers. In particular, best results are obtained with acrylonitrile, the dispersions of which are homogeneous and stable over time. On the contrary, in the case of vinyl chloride, the obtained dispersions do not show such favorable properties; in fact, owing to the morphology of the particles, the dispersions have a creamy consistency, or contain lumps, so that filtering is necessary before they are treated with the organic polyisocyanate.
From Italian Patent No. 1,152,478 homogeneous and stable dispersions of polyvinylchloride in polyol-polyether are known, obtained by polymerizing vinyl chloride in the liquid phase, and up to a conversion not higher than 40%, without stirring, or with a mild stirring, in the presence of an organic compound which acts as a solvent for monomeric vinyl chloride and as a precipitating agent for polyvinylchloride, and of a polyol having a molecular weight not higher than 10,000.
However, this process also has drawbacks, mainly due to the need of recovering the excess of unreacted monomer which is known to cause not only economic but also safety problem, due to the possible formation of unstable peroxides.
Stable dispersions of one or more thermoplastic polymer(s) or copolymer(s) such as polyvinylchloride in polyols are disclosed in French patent No. 2,284,638. According to this patent, stable dispersions are prepared by homogeneously blending the polyol, having a hydroxyl index within the range of from 10 to 1,000, with a latex of the thermoplastic polymer or copolymer having a particle size within the range of from 0.02 to 0.8 micrometers. After homogenizing the blend, the water supplied by latex is evaporated under vacuum at a temperature lower than 110.degree. C. The major drawback of the dispersions obtained by means of this process is that they are unstable in the formulations generally used for preparing polyurethanes. In fact, it was observed that the particles of polyvinylchloride separate from the dispersion and form lumps which cause drawbacks in the subsequent polyurethane processing.